(1) Field of the Invention
The present invention relates to an aromatic polyimide composition. More particularly, the present invention relates to an aromatic polyimide composition in which a high molecular weight aromatic polyimide prepared from an acid component comprising as a main acid ingredient, 3,4,3',4'-biphenyltetracarboxylic acid and an amine component comprising, as a main amine ingredient, 4,4'-diaminodiphenylether is dissolved in a naphthol or resorcin type mixed solvent, and which exhibits an excellent long storage stability and is useful for producing shaped articles having excellent mechanical properties and electrical properties.
(2) Description of the Related Art
It is known that an aromatic polyimide is produced by a two-step process. In the process, an aromatic tetracarboxylic acid component is polycondensed with an aromatic diamine component in an organic polar solvent at a relatively low temperature of, for example, 30.degree. C. or less, to provide a polyamic acid dissolved in the organic polar solvent, and, then, the polyamic acid is converted to a corresponding aromatic polyimide.
Recently, it has been learned that the aromatic polyimide can be produced in a single-step process wherein a specifically selected aromatic tetracarboxylic acid component and a specifically selected aromatic diamine component are polycondensed and imidized in one step in an organic polar solvent at an relatively high temperature. For example, U.S. Pat. Nos. 3,666,709 and 4,247,443 and Japanese Examined Patent Publication (Kokoku) Nos. 46-17145, 47-23191, 47-26878, and 48-26958 disclose process for producing aromatic polyimide solutions in which an aromatic acid component comprising, as a main acid ingredient, 3,4,3',4'-benzophenonetetracarboxylic dianhydride is reacted with an aromatic diamine component in the same molar amount as that the aromatic acid component, in a phenol solvent at an elevated temperature to polycondense and imidize in a single step.
Also, U.S. Pat. No. 3,988,303 and Japanese Unexamined Patent Publication (Kokai) No. 51-81899 disclose single-step processes for producing an aromatic polyimide solution, in which process an acid component comprising, as a main acid ingredient, 3,4,3',4'-benzophenonetetracarboxylic dianhydride or 1,4,5,8-naphthalenetetracarboxylic dianhydride and an aromatic amine component are polycondensed and imidized in a single step.
Furthermore, Japanese Examined Patent Publication (Kokoku) No. 47-37706 discloses a polyimide resin composition in which an aliphatic tetracarboxylic acid component is used.
When various shaped articles, for example, films, are prepared from the polyimide compositions or solutions as disclosed in the above-mentioned prior art, the resultant products exhibit unsatisfactory mechanical properties, for example, a very poor tensile strength of 10 to 15 kg/mm.sup.2 in comparison with those of conventional polyimide products (films) produced from polyimide solutions produced by the two-step polycondensation-imidization reaction of a tetracarboxylic acid component, for example, pyromellitic dianhydride with an amine component, for example, 4,4'-diaminodiphenylether.
Japanese Unexamined Patent Publication (Kokai) No. 50-113,597 discloses a process for producing a solvent-soluble aromatic polyimide from 3,4,3',4'-biphenyltetracarboxylic acid or 2,3,3',4'-biphenyltetracarboxylic acid. This aromatic polyimide is dissolved in a phenolic solvent, for example, m-cresol or xylenol, to provide a varnish useful for coating electric wires or cables. However, the resultant coating film exhibits unsatisfactory mechanical properties.
U.S. Pat. No. 4,290,936 discloses a solution of a polyimide composition useful for producing shaped polyimide articles, for example, polyimide films, having excellent mechanical properties. This type of aromatic polyimide composition solution is produced by polycondensing and imidizing an aromatic acid component comprising 80 molar % or more of 3,4,3',4'-biphenyltetracarboxylic acid with an aromatic amine component comprising 70 molar % or more of 4,4'-diaminediphenylether, in a solvent consisting of a halogenated phenolic compound at a temperature of 100.degree. C. to 300.degree. C. According to Example 2 of U.S. Pat. No. 4,290,936, the resultant polyimide film produced from a polyimide solution having a logarithmic viscosity of 2.38 exhibits an excellent tensile strength of 21.4 kg/mm.sup.2 and an ultimate elongation of 98.0%. However, the polyimide films or other shaped articles produced from a solution of the polyimide in a solvent containing a halogenated phenolic compound, for example, 4-chlorophenol, 4-bromophenol, or 4-fluorophenol, contain a small amount of the halogen compounds even after the solvent is removed from the films or the other articles at an elevated temperature. In certain uses of the aromatic polyimide composition, for example, high precision electric circuit boards in electronic devices, the halogen compounds are strictly prohibited, because they cause the electrical properties of the electronic devices to be degraded. Also, m-nitrophenol and catechol are usable as solvents for dissolving therein 5% by weight of a high molecular weight aromatic polyimide resin. However, when the m-nitrophenol is used, the cost of the resultant composition becomes undesirably high, because of the high price of m-nitrophenol and the difficult the removal and recovery of m-nitrophenol from the resultant shaped product by means of distillation. Also, when catechol is used, the resultant shaped products exhibit unsatisfactory mechanical and electrical properties, because of the easy decomposition of catechol.
It is strongly desired in the industry to obtain an aromatic polyimide composition which is completely free from the above-mentioned disadvantages.